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Plastic fantastic – new frontiers for recyclable polymers

Many modern plastics, rubbers and ceramics cannot be recycled, but new polymers made from waste sulfur are promising to solve one of the planet’s biggest recycling problems – and even create new industries of the future.

Researchers around the world have taken the next step to develop a range of these versatile and recyclable materials by controlling and improving their physical and mechanical properties to make them closer to scale up for manufacture.

Sulfur polymers are already being used in next-generation batteries, IR imaging (such as night-vision lenses), environmental remediation, and agriculture, but it has been difficult to control the hardness, flexibility, colour and other key properties of these polymers.

A new study reported in Chemistry – A European Journal identifies design principles that can better control the properties of these polymers, making them more adaptable and attractive for other types of manufacturing.

“Controlling their properties takes a big step towards these new polymers being able to replace plastics, rubber and ceramics that are currently unrecyclable,” says Flinders University’s Dr Justin Chalker, co-director of the study.

“Polymers made from elemental sulfur have emerged as versatile materials for energy storage, optics applications, environmental remediation and agriculture. It’s therefore critical that we establish design principles to control the thermal, mechanical and optical properties of these materials. This study provides a foundation stone for this endeavour.”

The research team’s goal was to establish, in a predictive way, which chemical building blocks impart which properties to the polymer.

For instance, making a polymer from sulfur, used canola oil and another low-cost industrial byproduct dicyclopentadiene (DCPD) can create several different types of materials. Add more canola oil, and it becomes a soft rubber. Add more of the rigid dicyclopentadiene molecule and the material becomes harder and more durable.

Through this research, the team was able to establish the precise compositions of monomers that impart a desired property.

For example, various polymer made from sulfur, limonene (a byproduct of the citrus industry) and DCPD at different ratios resulted in durable materials varying from soft waxes to hard glass all easily moulded into various shapes.

As another example, adding terpinolene (an essential oil found in allspice and other plant products) to the sulfur polymer turned from orange to red to black while the DCPD more shape persistent at higher temperatures.

“It’s exciting that we are able to use cheap and renewable resources to produce materials with unique properties,” says lead researcher Jessica Smith, from the University of Liverpool. “There are over 60 million tonnes of sulfur being produced by the petrochemical industry each year, and we can use other readily available household items, such as canola oil and linseed oil.

“Through this collaboration we have managed to investigate the mechanical properties and create a library of sulfur polymers. This sets a benchmark for other researchers to improve the physical properties further and refer to our research to choose particular polymers based on their mechanical strength.”

These discoveries are important, with new optics applications emerging for sulfur polymers as lenses and optical filters in infra-red thermal imaging, night-vision lenses and LIDAR surveying methods.

The researchers also found the new polymers can be broken down and reformed into new materials, representing a new era in recyclable materials made from renewable building blocks such as plant oils and industry byproducts such as sulfur.

“This research is another important set towards taking sulfur-based polymers in the lab towards a very useful, practical material that can impact our every-day lives,” say University of Liverpool collaborator Dr Tom Hasell.

Being able to produce polymers from sulfur – a waste product of the petrochemical industry – “is a really exciting opportunity”, both for the environment and for creating more sustainable products and industries.

“Almost every household item has some kind of polymer (plastic) in them and making polymers from sulfur not carbon opens doors into a new frontier of possibilities, with some applications yet to be found,” says Dr Hasell.

“So far these new polymers have potential applications in lenses for thermal imaging, making high-capacity batteries more stable to repeated charge/discharge, and filters to remove toxic heavy metals from water.

“With sulfur is so readily available, sulfur polymers could also find application in bulk applications in construction, especially where thermal insulation, electrical insulation, and chemical resistance are important.

But first the research team is working to understand and manage the new polymers’ physical properties to prepare them for new and emerging commercial and industrial applications.

Background: This study was a collaboration between Dr Tom Hasell at the University of Liverpool and Dr Justin Chalker at Flinders University Institute for NanoScale Science and Technology. Liverpool University researcher Jessica Smith carried out a portion of this work at Flinders University – while Flinders PhD student Max Mann is visiting the Hasell laboratory – assisted by Royal Society-funded student exchange funding.

Other contributors to the study were researchers from the University of Birmingham and Loughborough University in the UK. Max Worthington, Dr Christopher Gibson, Associate Professor Jonathan Campbell, and Professor David Lewis from the Flinders Institute for NanoScale Science and Technology, were also co-authors of the study.